False Bay's waters and sediments contain a cocktail of pharmaceuticals, pesticides and metals linked to urban development, wastewater discharges and harbour activities, according to a new study led by researchers from the University of Cape Town (UCT).
The study, published in Marine Pollution Bulletin, investigated the presence and ecological effects of selected pharmaceutical compounds, herbicides and metals across 19 sampling stations in False Bay between April and June 2021. Researchers detected pharmaceuticals such as acetaminophen, carbamazepine and diclofenac, along with the herbicides atrazine and metolachlor, in both seawater and marine sediments throughout the bay.
The findings revealed that contaminant concentrations were generally much higher in sediments than in seawater, suggesting that the seabed acts as a long-term storage reservoir for pollution entering the bay.
"We found that pharmaceuticals and herbicides are widespread throughout False Bay and that sediments appear to retain these contaminants over extended periods," said Dr Buyani Mazeka, the study's lead researcher and a postdoctoral fellow at UCT's Department of Biological Sciences.
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The highest seawater contaminant loads were recorded in the northeastern sector of the bay near Gordon's Bay, while the highest sediment accumulations were found in areas near Muizenberg and parts of the northeastern coastline.
Diclofenac, a commonly used anti-inflammatory medicine, was among the most frequently detected pharmaceutical compounds. Dr Mazeka noted that its persistence in the environment and continued input through wastewater systems make it a contaminant of concern.
The study also measured concentrations of copper, iron, manganese, lead and zinc in sediments. Elevated metal levels were found in areas influenced by urbanisation, wastewater discharges and harbour-related activities. Sites near Gordon's Bay showed particularly high concentrations of some metals, which researchers linked to maritime activities, vessel maintenance operations and land-based pollution sources.
Little evidence of ecological damage
Despite the widespread presence of contaminants, the researchers found little evidence that current pollution levels are causing immediate ecological damage.
Environmental risk assessments showed that risk quotient values remained well below internationally recognised thresholds of concern. Metal concentrations were also generally below levels associated with adverse biological effects.
Importantly, benthic communities - organisms living on and within the seafloor - remained diverse and healthy across the sampling sites.
The research team examined both larger bottom-dwelling animals and microscopic nematodes, which are widely used as indicators of ecosystem health. Their analysis found no significant relationship between contaminant concentrations and measures of abundance, diversity or community structure.
"The concentrations measured during this study were generally insufficient to produce consistent changes in benthic community structure."
Nematode indicators further suggested that most sampling sites maintained good to high ecological quality. "The concentrations measured during this study were generally insufficient to produce consistent changes in benthic community structure," said Mazeka.
However, Mazeka and his colleagues caution that the absence of immediate impacts should not be interpreted as evidence that the contaminants are harmless. While acute ecological effects appear unlikely at current concentrations, the study highlights uncertainties around long-term exposure, contaminant mixtures and substances that were not included in the analysis.
"Pharmaceuticals can accumulate in sediments and may be transferred through marine food webs," Mazeka explained. Previous research in False Bay has already demonstrated uptake of some pharmaceutical compounds by marine organisms, including mussels, limpets, sea snails, sea urchins and starfish.
"We therefore describe pharmaceutical residues as contaminants of emerging concern that warrant continued attention," shared Mazeka.
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First study of its kind
This is the first field-based baseline study of its kind in Africa and among the first in the Southern Hemisphere. Unlike many previous studies, which have relied on microcosm experiments, this work is based on field observations and simultaneously quantified selected pharmaceuticals, herbicides and metals, while using both macrofauna and nematodes as complementary bioindicators to assess their potential ecological effects.
False Bay is one of South Africa's most important coastal ecosystems and supports high biodiversity, fisheries, tourism and recreational activities. At the same time, it receives inputs from wastewater treatment plants, stormwater systems, rivers and urban development along its coastline.
The study recommends long-term monitoring programmes, more comprehensive contaminant screening and higher-resolution biological assessments to improve understanding of cumulative ecological risks.
